KARST MANAGEMENT IN FARNORTH WESTERN TASMANIA
INTRODUCTION
Limestone and dolomite of upper Precambrian - lower Cambrian age crops out extensively in far northwestern Tasmania, together with more restricted occurrences of marine limestone of Tertiary age and Pleistocene freshwater limestones that form mound springs (Williams and Turner 1973). The Palaeozoic carbonates occur predominantly as broad areas of low relief, the intervening ridges being formed of younger non-carbonate sedimentary rocks.
Although karst caves have been recorded from a number of localities the area is probably most noteworthy for possessing a range of surface karst landforms that are of nature conservation significance in their own right. In addition, the karst has profoundly influenced patterns of settlement and human use of the area. The low relief areas, locally known as swamps, have been extensively drained for agricultural use and represent some of the most productive dairy pasture in the world. Forestry activity has occurred on some of the ridges in the northern part of the area since the 1920s and all the swamps have been subject to at least one cycle of logging. Today logging is most heavily focussed in some of the more remote areas further to the south. This paper briefly reviews management of the karst in this area, particularly in areas of State Forest.
THE KARSTS
Knowledge of the karsts of far northwestern Tasmania is far from complete but a broad overview of the area has recently been completed by Henriksen (1990) on contract to the Forestry Commission. Apart from the cleared swamps and the narrow coastal fringe, much of the area is cloaked in primary forest or dense re-growth, and the area is sufficiently distant from Tasmania's principal population centres as to have inhibited the activities of recreational cavers. Academic research has focused on Pleistocene bone deposits that have been located in several areas (Murray and Goede 1977), on the palynological record in Pleistocene mound-springs, and the history of changes in the relative levels of land and sea during the late Cainozoic (van der Geer & Colhoun 1977). Given sea level decline by in excess of 100m during glacial climatic stages, last Interglacial sea levels 20m above present, and continuing uplift that implies much of the area having lain below sea level during the late Cainozoic, some of the karst towards the coast probably evolved through phases conducive to rapid karstification that have alternated with periods of inundation and mantling of the carbonate rocks by marine sediment.
Some of the most interesting of the karsts in the area include:
- Dismal Swamp. This broad depression has the form and structure of a karst polje (Kiernan 1990). It is one of only three poljes recorded from Tasmania and more nearly approaches a classical polje than either of the other two both of which are karst-margin features. Brittons Swamp a few kilometres to the north-east of Dismal Swamp may be of related origin but has been extensively modified during the course of pasture development.
- Montagu. Two small limestone hills that rise above the alluvial flats in this area contain numerous small caves that are best known for fossil bone deposits that include extinct Pleistocene megafauna (Kiernan 1973; Murray and Goede 1977). The caves also contain a reasonable assemblage of speleothems.
- Redpa. In this area a series of small limestone outcrops exhibit well-developed swamp notches and related features, together with one of the best developed bare rock karren assemblages of any Tasmanian inland karst. One larger hill contains numerous small caves some of which are interconnected (Kiernan 1974). The surrounding flat land has been cleared for pasture.
- Julius River. The Julius River passes underground through a small ridge of limestone in this area, leaving its former course 'dry'. A surface tributary that enters the 'dry' former channel midway along its length splits into two anabranches that flow in opposite directions, one to the sink of the Julius and one to its resurgence. Detailed exploration and study of the caves in this area is yet to occur (Kiernan 1980).
- Lake Chisholm. Close by the Julius River caves lies the beautiful sinkhole lake known as Lake Chisholm. The area also contains numerous solution sinkholes and cover-collapse sinkholes.
- Trowutta Arch. At this locality an impressive natural arch separates two large collapse dolines. The neighbouring area has not been explored in detail for caves although some small caves are rumoured to occur (Kiernan 1974). A deep pool in one of the sinkholes has created some interest among cave divers (Hume 1983). This site is also of considerable biospeleological interest for its aquatic fauna (Eberhard et al 1991).
- Farnhams Creek. At least three subjacent karst collapse dolines have formed in non-carbonate Cambrian sedimentary rocks that overlie the main dolomite formation but may themselves contain thin beds and lenses of limestone. The largest of the dolines are sheer-sided, up to 30m in diameter and 15m deep. Two are water-filled.
- Northern Coastal Area. A noteworthy feature of the coastal fringe between Pulbeena, Smithton and Mella is the existence of a series of mound springs, very few of which are known from elsewhere in Tasmania. The mounds rise up to 10m above the plains and in some cases the temperature of the water that discharges from them is elevated a few degrees above that of surrounding surface waters.
MANAGEMENT AND MANAGEMENT ISSUES
The low relief 'swamps' lend themselves readily to development for agricultural purposes although capital injection is necessarily high to create a drainage system and some of these areas are still highly waterlogged during winter. Despite these economic advantages, the integrity of most of the karst in the agricultural areas has been severely compromised by hydrological changes. Most of the known mound springs have been modified in the course of pasture development and in some cases by quarrying. The quarrying of limestone also contributes to the local economy, but in addition to damaging mound springs has resulted in the removal of at least some important features, including a cave at Scotchtown that contained an assemblage of Pleistocene bone-beds. Groundwater from the carbonate aquifers is a locally significant resource (Guiline 1959).
Timber production has long been important to the economy of this part of Tasmania. While extensive areas have been logged the environmental impact has not been as great as in the agricultural areas and extensive tracts of virgin forest remain. Hence, in the forested areas the opportunity still exists to safeguard some of the natural karst values of the area. The Tasmanian Forest Practices Code includes specific guidelines designed to minimise adverse impacts on karst environments. One significant issue in the areas of production forest is that infiltration occurs more readily into the carbonate rocks than into many of the other substrates in the area. Coupled with the presence of a chert cap on the ridges, this has led to the development of thinner, more gravelly regolith than on most of the other rock types. Hence, the karsts have been favoured for winter logging, which has probably facilitated the entry of turbid water into the aquifers and some potential destabilisation of the regolith due to cavity formation.
The future of any logging of Dismal Swamp is currently being considered by the Forestry Commission. A number of formally protected areas contain karst phenomena. A small State Reserve has been designated at the Trowutta Arch, and another covers a small part of Dismal Swamp. Forest Reserves have been established to protect karst features at the Julius River caves and at Lake Chisholm. Some karst also occurs in the Balfour Track Forest Reserve. The Lake Chisholm reserve suffered some damage from a fire lit in neighbouring terrain for forest management purposes a few years ago. The reserve at the Trowutta Arch has also been compromised by the dumping of debris into the northernmost sinkhole and pond following pasture clearing on the margin of the reserve. APPM Forest Products has recently established a company reserve over the caves at Montagu, provides interpretive material that seeks co-operation with conservation priorities from those seeking access to the area, and proposes to install in-cave signs to minimise the risk of bone deposits being trampled.
DISCUSSION
The karst areas of far northwestern Tasmania offer important economic resources in terms of their agricultural, forestry and mineral potential. They are also important scientific resources that offer scope for important studies in a range of disciplines. Few of these scientific opportunities have as yet been exploited to any degree but they have the potential to produce studies of very major importance some of which will undoubtedly be of significance far beyond the immediate local area. The karsts of this area contain surface landform species unknown elsewhere in Tasmania and in at least one case probably not replicated elsewhere in Australia. They also offer important recreational opportunities.
Achieving an acceptable balance between the competing demands requires a sensitivity to the existence of these diverse values and ongoing survey of the karsts to enable important or difficult areas to be identified well in advance of operational activities. The Forest Practices Code facilitates the protection of features in areas of production forest and some reserves have also been established. However, agricultural activities have resulted in very severe impacts to some of the most important features and until such time as a code of practice along the same lines as the Forest Practices Code is in place a continuation of these impacts is likely to be constrained only by the nature of the topography.
ACKNOWLEDGEMENTS
We acknowledge the support of the Tasmanian Karst Atlas Project by the Tasmanian Forest Research Council.
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